The present invention relates to an improved spatial light modulator ("SLM"), and, more particularly, to an SLM of the digital micromirror device ("DMD") variety having improved operating characteristics.
SLM's are transducers that modulate incident light in a spatial pattern pursuant to an electrical or other input. The incident light may be modulated in phase, intensity, polarization or direction. SLM's of the deformable mirror class include micromechanical arrays of electronically addressable mirror elements or pixels which are selectively movable or deformable. Each mirror element is movable in response to an electrical input to an integrated addressing circuit formed monolithically with the addressable mirror elements in a common substrate. Incident light is modulated in direction and/or phase by reflection from each element.
As set forth in greater detail in commonly assigned U.S. Pat. No. 5,061,049, deformable mirror SLM's are often referred to as DMD's (for "Deformable Mirror Device" or "Digital Micromirror Device"). There are three general categories of deformable mirror SLM's: elastomeric, membrane and beam. The latter category includes torsion beam DMD's, cantilever beam DMD's and flexure beam DMD's.
Each movable mirror element of all three beam types of DMD's includes a relatively thick metal reflector supported in a normal, undeflected position by an integral relatively thin metal beam. In the normal position, the reflector is spaced from a substrate-supported, underlying control electrode which may have a voltage selectively impressed thereon by the addressing circuit.
When the control electrode carries an appropriate voltage, the reflector is electrostatically attracted thereto and moves or is deflected out of the normal position toward the control electrode and the substrate. Such movement or deflection of the reflector causes deformation of its supporting beam thereby storing therein potential energy which tends to return the reflector to its normal position when the control electrode is de-energized. As a practical matter may be, and often is, insufficient to return the reflector to the normal position. This necessitates the application of one of a variety of reset signals or voltages between the control electrode and the reflector to achieve this end.
The deformation of a cantilever beam comprises bending about an axis normal to the beam's axis; that of a torsion beam comprises deformation by twisting about an axis parallel to the beam's axis; that of a flexure beam, which is a relatively long cantilever beam connected to the reflector by a relatively short torsion beam, comprises both types of deformation, permitting the reflector to move in piston-like fashion. Thus, the movement or deflection of the reflector of a cantilever or torsion beam DMD is rotational, with some parts of the reflector rotating toward the substrate and other parts rotating away from the substrate if the axis of rotation is other than at an edge or terminus of the reflector. The movement or deflection of the reflector of a flexure beam DMD maintains all points on the reflector generally parallel with the substrate.
When the reflector of a beam DMD is operated in binary fashion by its addressing circuit, it occupies one of two positions, the first being the normal position which is set by the undeformed beam, the second position being a deflected position. In one of the positions, the reflector reflects incident light to a selected site, such as a viewing screen, the drum of a xerographic printer or other photoreceptor. In the other position, incident light is not reflected to the photoreceptor.
A typical DMD includes an array of numerous pixels, the reflectors of each of which are selectively positioned to reflect or not reflect light to a desired site.
Because a potential difference must exist between the reflector and the control electrode to deflect the reflector, it is undesirable for these two elements to engage. Engagement of a deflected reflector and its control electrode effects current flow therethrough which may weld them together and/or cause the thinner beam to melt or fuse. In either event the functionality of the involved pixel is destroyed. In response to the foregoing problem, a landing electrode may be associated with each reflector. Typically, in the case of a cantilever- or torsion-beam DMD, the landing electrode resides on the substrate at a greater distance from the rotational axis than the control electrode, both distances being taken parallel to the reflector in its normal position. In a flexure-beam DMD, the top of the landing electrode is elevated above the top of the control electrode. In view of the foregoing, the deflected reflector ultimately engages the landing electrode, but not the control electrode. To prevent damage to reflector, the landing electrode is maintained at the same potential as the reflector. Again, see commonly assigned U.S. Pat. No. 5,061,049.
Notwithstanding the use of a landing electrode, it has been found that a deflected reflector will sometimes stick or adhere to its landing electrode. Such sticking or adherence prevents the energy stored in the deformed beam, or reasonable forces applied to the reflector in other ways, from returning or "resetting" the reflector to its normal position after the control electrode is deenergized. It has been postulated that such sticking is caused, inter alia, by (a) welding (b)intermolecular attraction between the reflector and the landing electrode or (c) high surface energy substances or other particulate, liquid or gaseous contaminants sorbed or deposited on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances which impart high surface energy to the reflector-landing electrode interface include water vapor and other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, hydrogen, nitrogen) and gases and organic components resulting from or left behind following production of the DMD, including gases produced by outgassing from UV-cured adhesives which mount a protective cover to the DMD. Such a protective cover and other DMD "packages" are disclosed in commonly assigned U.S. patent application Ser. No. 033,687, filed Mar. 16, 1993.
Sticking of the reflector to the landing electrode has been overcome by applying selected numbers, durations, shapes and magnitudes of voltage pulses (the previously noted "reset signals") to the control electrode. One type of reset signal attempts to further attract toward the landing electrode a reflector which already engages the landing electrode. This further attraction stores additional potential energy in the already deformed beam. When the control electrode is de-energized, the potential energy stored in the beam is now able to unstick the reflector from the landing electrode and return the reflector to its normal position. A variant reset signal comprises a train of pulses applied to the control electrode to induce a resonant mechanical wave in a reflector already engaging a landing electrode. De-energizing the control electrode as a portion of the reflector is deformed away from the landing electrode unsticks the reflector. For more details concerning the foregoing and other unsticking techniques, see commonly assigned U.S. Pat. No. 5,096,279.
Sticking or adherence of the reflector and the landing electrode may be reduced by appropriate liquid lubricants. Moreover, in commonly assigned U.S. Pat. No. 5,331,454, there are disclosed techniques for passivating the portion of the landing electrode engaged by the deformed reflector and/or the portion of the deformed reflector which engages the landing electrode so that sticking or adherence therebetween is reduced or eliminated. Passivation may be effected by lowering the surface energy of the landing electrode and/or the reflector--or otherwise preventing sticking or adhering. Passivation may be, in turn, effected by chemically vapor-depositing on the engageable surfaces of interest a monolayer of a long-chain aliphatic halogenated polar compound, such as a perfluochemical, examples of which are perfluoroalkyl acid, perfluorodecanoic acid (PFDA), perfluoropolyether (PFPE) and polytetrafluoroethylene (Teflon).
The polar compound perfluoroalkyl acid comprises a chain having an F.sub.3 C molecule at a first end, a COOH molecule at the second end and intermediate CF.sub.2 molecules. The COOH end becomes firmly attached to surfaces of the DMD--following pretreatment, if necessary, to achieve same--to present very low surface energy F.sub.3 C and CF.sub.2 molecules for engagement. The other materials function similarly.
The application of the foregoing a compounds to at least that portion of the landing electrode which is engaged by a deformed reflector has resulted in an amelioration of the sticking or adhesion problem.
Objects do not easily, if at all, stick or adhere to low surface energy surfaces. Further, sticking or adherence of substances to low energy surfaces should not occur or be minimized since such surfaces should be resistant to sorption thereonto of the above-discussed high-surface-energy-imparting substances, such as water vapor. Indeed, DMD's on which an anti-stick monolayer, lubricant or other appropriate substance has been deposited may initially exhibit little if any reflector-electrode adherence. This is evidenced by the low magnitudes of reset signals and/or by the proper functioning over time of all or a maximal number of reflectors.
After the DMDs are operated for some time, however, two effects have been noted. First higher magnitudes of reset signals may be required to return the reflectors to their normal positions. Second, at a given reset voltage less than all or a maximal number of the reflectors may return to their normal positions. The same two effects have been noted when protective, lighttransparent covers are mounted to DMD's with adhesives, such as UV-cured epoxies. The above effects have also been noted as worsening after extended operation of DMDs. The foregoing suggests that substances deposited or outgassed from the ambient, from adhesives or from the DMD itself are somehow adhering to, becoming incorporated into or otherwise adversely affecting the low surface energy anti-stick deposit or lubricant, possibly due to defects or discontinuities in the long chains, monolayers or other structure thereof.
Elimination of the sticking phenomenon is an object of the present invention.